1. Field of the Invention
This invention relates to an apparatus and method for measuring the particle number rate and the particle velocity distribution of a sprayed stream such as, a shot peening system. In particular, a double spinning disk apparatus modified with a stationary aperture and a scanning aperture is used to scan the spayed stream and observe the stream's local properties.
2. Description of the Related Art
When applying any spray deposition process, uniformity of the deposited layer is of key importance. Since spray streams have a pattern geometry, generally round, and do not generally have uniform properties over the pattern area, it is necessary to overlap adjacent spray passes in a manner that gives uniform thickness to the finished sprayed layer.
This applies even in the simplest case of a circular spray stream having uniform properties. For example, consider depositing a uniformly thick layer with a circular spray stream that has uniform properties over its entire cross-section. In this case the deposition for a pass is proportional to the time the pattern resides over a point. Thus, the thickness at a point will vary as the cordal length of the spray circle. Therefore, the layer for a individual pass will be thickest at the center of the pattern and drop off to zero at its edges. If a surface is sprayed by such a stream and the pattern for adjacent passes just touch, the final layer depth will be as shown in FIG. 1, where R is the radius of the spray pattern plotted against distance (d) on the pattern. On the other hand, if the layer is made up of two equally spaced, overlapped passes, the result will be that shown in FIG. 2 where radius (R) is plotted against distance (d) on the pattern. For the pattern of FIG. 1 there are areas where the deposition depth is zero. For obvious reasons, this does not happen for FIG. 2. In FIG. 2, the ratio of the thickest to thinnest deposition is 1.73.
If the overlap is increased from 2 to 3, this ratio decreases to 1.19 and a relatively uniform deposition depth starts to appear. As the number of overlaps increases, the uniformity of the deposited layer gets better and better.
Consider next what happens when there is a distribution of properties over the spray stream. In the simplest case of a circular stream, the properties are axisymmetric and vary only with the radius from the center of the stream.
In this case, the argument made for the exposure time (based on cordal length) applies, but now the spray stream density as a function of cordal position must be included and the total deposition for each cord integrated. In this way, the same logic used for the simplest, uniform case can be applied and the conditions for the desired surface deposition depth established.
The arguments made above apply to depositions such as paint spray and plasma deposition. In the case of shot peening, the problem, although treated in a similar way, is more complex because two spray stream distributions act simultaneously.
In shot peening operations, not only is the number rate density important since, like paint spray, it effects coverage rate. The particle velocity distribution is also important because it effects the intensity of the compressive layer as a function of stream location. Of course, this effect can be dealt with using logic like that already described for deposition depth. However, before accounting for either effect (coverage or intensity) it is vital that a means be available to measure the distribution of properties in the spray stream.
It is apparent from the above that there exists a need in the art for a single instrument which is capable of measuring the velocity distribution and the particle rate density in a sprayed shot stream. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.